There is an increasing demand on mobile wireless operators to provide voice and high-speed data services, and at the same time, these operators want to support more users per base station to reduce overall network costs and make the services affordable to subscribers. As a result, wireless systems that enable higher data rates and higher capacities to the user equipment are needed. The available spectrum for wireless services is limited, however, and the prior attempts to increase traffic within a fixed bandwidth have increased interference in the system and degraded signal quality.
Wireless communications networks are typically divided into cells, with each of the cells further divided into cell sectors. A base station is provided in each cell to enable wireless communications with mobile stations located within the cell. One problem existing in the prior art systems includes the situation where the transmission/reception of each user's signal becomes a source of interference to other users located in the same cell location on the network, making the overall system interference limited.
An effective way to increase efficiency of bandwidth usage and reduce this type of interference is to use multiple input-multiple output (MIMO) technology that supports multiple antennas at the transmitter and receiver. For a multiple antenna broadcast channel, such as the downlink on a cellular network, transmit/receive strategies have been developed to maximize the downlink throughput by splitting up the cell into multiple sectors and using sectorized antennas to simultaneously communicate with multiple users. Such sectorized antenna technology offers a significantly improved solution to reduce interference levels and improve the system capacity.
The sectorized antenna system is characterized by a centralized transmitter (cell site/tower) that simultaneously communicates with multiple receivers (user equipment, cell phone, etc.) that are involved in the communication session. With this technology, each user's signal is transmitted and received by the basestation only in the direction of that particular user. This allows the system to significantly reduce the overall interference in the system. A sectorized antenna system consists of an array of antennas that direct different transmission/reception beams toward each user in the system or different directions in the cellular network based on the user's location.
To improve the performance of a sectorized cell sector, schemes have been implemented using orthogonal frequency domain multiple access (OFDMA) systems. The various components on the system may be called different names depending on the nomenclature used on any particular network configuration or communication system. For instance, “user equipment” encompasses PC's on a cabled network, as well as other types of equipment coupled by wireless connectivity directly to the cellular network as can be experienced by various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like.
Further, the words “receiver” and “transmitter” may be referred to as “access point” (AP), “basestation,” and “user” depending on which direction the communication is being transmitted and received. For example, an access point AP or a basestaion (eNodeB or eNB) is the transmitter and a user is the receiver for downlink environments, whereas an access point AP or a basestaion (eNodeB or eNB) is the receiver and a user is the transmitter for uplink environments. These terms (such as transmitter or receiver) are not meant to be restrictively defined, but could include various mobile communication units or transmission devices located on the network.
One of the main challenges faced by the current system developers is providing high throughput at the cell edge. Technologies like multiple input multiple output (MIMO), orthogonal frequency division multiplexing (OFDM), and advanced error control codes enhance per-link throughput, but these technologies do not solve the detrimental effects of interference at borders with other cells or at the cell edge.
Cell edge performance is becoming more important as cellular systems employ higher bandwidths with the same amount of transmit power, and the systems use higher carrier frequencies with infrastructure designed for lower carrier frequencies. New standards are needed for mobile broadband access that will meet the throughput and coverage requirements of a fourth generation cellular technology.